1. Field of the Invention
This invention relates to a method and apparatus for optically measuring marine conditions, and particularly for providing a measure of the number and size of marine organisms or particles.
2. Description of the Prior Art
Fisheries and research scientists worldwide require continuous information/data on the marine food chain which dominantly consists of: phytoplankton (typically 1 to 10 .mu.m in diameter, esd (equivalent spherical diameter)); zooplankton (typically 100.mu.m to 20 mm, esd); and fish, where each becomes a food source for the next in ascending order. Zooplankton can be classified in two sub-categories of size: microzooplankton (typically 50 to 250 .mu.m, esd); and macrozooplankton (typically 250 .mu.m to 20 mm, esd). Fish can also be sub-categorized in size: fish eggs (typically 1 mm in diameter); fish larvae (typically 1 to 3 cm in length); and juvenile and adult fish of sizes larger than their larval stage. Phytoplankton and zooplankton are measured in the oceans and studied for their interrelationships and for their profound effect on fisheries. Information is required on their abundance and vertical distributions in continental shelf waters, deep oceans and inland waters. Acquiring this data accurately, continuously and with wide spatial coverage with limited ship-time is a major sampling problem.
In the past the sampling of zooplanklton has generally been accomplished by towing large plankton nets with mouth openings of approximately 0.5 to 2.0 meters and lengths of approx. 3-6 meters. Some remote sensors towed behind a ship such as conductivity cells and video cameras have also been used. Sampling phytoplankton has generally been accomplished by two methods: by taking water bottle samples and processing the water samples by fluorometric methods, and by lowering or towing electronic instruments such as a fluorometer or a light attenuance meter. The light attenuance meter is less accurate in measuring phytoplankton biomass than the fluorometer but does provide a reasonable profile of `relative biomass` when calibrated against a fluorometer at several points in sea water.
Deployment of zooplankton sampler nets from ships is generally cumbersome, time consuming and provides limited spatial coverage. The nets clog with phytoplankton material and must be recovered after short tows of approximately 10 minutes. Vertical information is generally lost, since all the sample is integrated in the net. Designs using of multiple stacked nets can yield improved but still very limited vertical information. Remote sensing of zooplankton using conductivity cells has not proven successful since the cells are small (3 to 5 nm in diameter) and unable to sample sufficient water volume.
Video cameras are suitable for imaging and identifying zooplankton but have considerable difficulty in processing in real-time because of the large volume of data, and are difficult to operate since they require spatial lighting, and can only be towed at slow speeds of 1-2 knots. Obtaining simultaneous data on zooplankton and phytoplankton and smaller fish larvae is often not done since it requires the addition of other instrumentation thereby increasing complexity and cost.
A common problem encountered by devices using light beams to count particles is the presence of 2 or more particles in the light beam resulting in a single count. The `coincidence` problem is dependent on both the size of the light beam and the density of particles in the water.
A method and apparatus for the optical measurement of marine conditions is disclosed in applicant's earlier U.S. Pat. No. 4,637,719. The prior device could not determine the shape of an organism for identification, and was capable of detecting a relatively narrow organism/particle size range, limited to organism size above 250 .mu.m, and could not determine the water speed. Also, the prior device was subject to the `coincidence` problem referred to above, and was sensitive to vibration at sizes less than 200 .mu.m.